feat(agents): use claude --prefill for Use direct-launch draft

[nwparker]

Summary

The "Use" CTA on the Task page creates a workspace and drops the GitHub issue/PR URL into the agent's input box as a reviewable draft (not submitted). Today that's done via a post-launch bracketed-paste after waitForAgentReady + a 120ms settle — it works, but there's a real race window.

Claude Code exposes --prefill <text>, which launches the TUI with the text already in the input box and nothing submitted. For Claude users this replaces the readiness + paste dance entirely while preserving the "draft, don't send" contract.

Changes

• src/shared/tui-agent-config.ts — add optional draftPromptFlag to TuiAgentConfig; set '--prefill' on claude only.
• src/renderer/src/lib/tui-agent-startup.ts — new buildAgentDraftLaunchPlan helper. Returns a launch command when the agent has a prefill flag, null otherwise.
• src/renderer/src/lib/launch-work-item-direct.ts — prefer the draft plan; skip the post-launch paste when one is used (pasting again would duplicate the URL). Every other agent keeps today's paste-after-ready path unchanged.
• Tests: prefill case for Claude, null fallback for agents without the flag, empty-draft fallback, and cmdOverrides honoring.

Agent CLI survey

Checked every CLI we could run locally (codex, cursor-agent, droid, gemini, copilot, pi, opencode, aider) — Claude is the only one with a documented "prefill but don't submit" flag today. gemini -i/--prompt-interactive and copilot -i/--interactive both auto-execute. The draftPromptFlag shape lets us drop in additional agents as one-liners when they ship the equivalent.

Test plan

• pnpm typecheck
• pnpm lint (no new warnings)
• pnpm exec vitest run src/renderer/src/lib/tui-agent-startup.test.ts — 14 tests pass
• Manual: on the Task page with Claude as default agent, click "Use" on a GH issue → Claude opens with the issue URL pre-filled in the input, not submitted
• Manual: repeat with a non-Claude default (codex/cursor/etc.) → behavior unchanged; URL still lands as a bracketed-paste draft after agent ready